Earlier this year, Zadie Smith visited the set of White Teeth, where her epic and slowly unfolding patchwork of a novel was being transformed into a taut and fast-moving four-part television drama. She turned up on a grey winter's morning, with some of her friends in tow, to take part in a crowd scene that was being shot somewhere in the furthest reaches of north London, way beyond even Willesden where most of the narrative is set.

'There were about 15 white trailers in a big car park,' she remembers, 'and all these technicians and extras everywhere. People were grumpy about having had to get up so early, and some of the extras were moaning about their costumes. It was a bit unnerving. It suddenly hit me that something I had done way back there was responsible for all these people being upset at eight in the morning. I suddenly felt really guilty and responsible. It was,' she says, laughing now at the memory, 'like the responsibility of having a business.'

Which, in one way, of course, is exactly what White Teeth has become - one of those runaway success stories that every first-time novelist dreams of, then has to retreat from in order to function again as a writer. When the rights were sold to television shortly after its publication in early 2000, White Teeth was simply a much-hyped, and relatively successful, first novel by a precociously talented 24-year-old, mixed-race Cambridge graduate who, flying in the face of the modern English literary novel as we know it, had dared to think big in terms of her subject matter. By the time shooting had started on Channel 4's adaptation earlier this year, the book had become a British publishing phenomenon, a sprawling and unapologetically literary novel of ideas that had sold more than a million copies. There was a moment, back there, when everyone seemed to be reading it, when every bookshop window seemed to display it.

When the four-part Channel 4 adaptation of White Teeth debuts on our screens next month, the book, buoyed by the renewed interest that only a high-profile television drama can provide, will once again start selling by the barrowload. When Zadie Smith talks about suddenly experiencing 'the responsibility of having a business', she is not referring to a cottage industry. 'I thought some people would get it and like it,' she tells me. 'But not how many, and for so long. There is,' she adds, with the air of someone who has long since ceased thinking about the whys and wherefores of her own popularity, 'no accounting for taste.'

One gets the distinct feeling too that it has not, perhaps unsurprisingly, been all plain sailing for the 26-year-old Smith. Her period of media ubiquity lasted from White Teeth's hardback publication in early 2000 to her winning of the Guardian First Book Award in December that same year. Reading through her press cuttings dating from the time, one gets the feeling that had Zadie Smith not come along, certain sections of the media - broadsheet rather than tabloid, for once - would have been forced to invent her. She seems at times to have been the product of a collective wish fulfilment by a British press which needed an attractive, young, black, female literary starlet to fulfil some notion of a newly progressive multiracial Britain where race riots, the BNP and the Stephen Lawrence murder were unfortunate aberrations.

With the lionising, too, came the inevitable bitching. She is still angry at a Guardian interview that described her as 'morose, self-conscious and resentful', and wherein the writer took personal issue with her change of hairstyle, and the fact that she no longer resembled the sexy, bespectacled young intellectual pictured on the hardback book jacket. In interview, and indeed in private, Smith can come across as both self-assured and gauche, an oddly English trait that may, or may not, be a product of her relocation from the cacophonous streets of Kilburn to the cloistered halls of Cambridge. This time around, though, perhaps wary of either another round of publicity overkill or the kind of backlash that often follows success of the sudden and unexpected kind, she has opted to be uncharacteristically tight-lipped on the subject of her new book, The Autograph Man. 'People can read the book, or not read the book,' she says matter-of-factly, 'and obviously I hope they do, but someone doesn't have to be the biggest best seller ever to be a good writer. That's not what it's about. Plus, I just don't want to have to give so much of myself away.'

To this end, she will be heading for Harvard as the book reaches the shops to pursue a postgraduate course on the modern European novel.

By coincidence - at least, that's what the publishers are saying - The Autograph Man will hit the shops around the same time as White Teeth hits the screens. If the success of the latter book said much, on one level, about the British public's curiosity about the changing nature of itself, The Autograph Man circles around the notion of contemporary celebrity and the obsession it inspires. It follows the fortunes of Alex Li-Tandem, a Chinese Jew who trades in celebrity autographs, both authentic and fake. According to Smith, it is both a whole lot funnier and sparser than its acclaimed predecessor, which she once memorably described as 'baggy'. A long extract from her latest book recently published in The New Yorker certainly bears this out, and suggests that, rather than succumbing to difficult second-novel syndrome, Smith has found her voice, shedding White Teeth's occasional reliance on convoluted plot machinations and creating a surfeit of eccentric characters.

The Autograph Man, on this showing at least, is pared down and pristine, funny in a way that seems altogether less overstated. The literary ambition remains, though. On one level, The Autograph Man is a quest novel, tracking Tandem as he pursues answers to questions both metaphysical and profane, including, as the blurb puts it, 'the return of his father, the reinstatement of some kind of all-powerful benevolent God-type figure, the end of religion, something for his headache, three different girls, infinite grace, and the rare autograph of silent-movie actress Kitty Alexander. With fries.'

Whatever the reasons for Smith's reticence in talking about the new novel, she must surely be relieved that she didn't succumb to one of those legendary bouts of writerly paralysis that struck the likes of Salinger and Donna Tartt in the wake of their acclaimed early masterpieces. 'I just feel a whole lot calmer, pathetically calm, in fact,' she says after an uncharacteristically long pause. 'It suddenly seems a long way away. That's what happens when you finish a book. When I think about White Teeth now, it just seems like an oddly distant thrill.'

In a leafy square in central London, I am experiencing the oddly distant thrill of watching a bunch of extras impersonating a gaggle of noisy Jehovah's Witnesses, brandishing placards scrawled with biblical slogans,and singing a hymn of protest loudly and tunelessly. A Channel 4 film crew, overseen by director Julian Jarrold, have been gathered here all morning to film the scene where long-time religious fanatic Hortense Bowden (played by Mona Hammond, who was Blossom in EastEnders) and her unlikely ally, Ryan Topps (Charlie Creed-Miles) - erstwhile Mod and erstwhile boyfriend of her wayward daughter, Clara - arrive outside the Millennium Science Commission to protest against the ungodly act of cloning.

Perhaps readers are more possessive of books and their characters than the authors who created them, but the scene I am witnessing seems both familiar and strangely wrong - at odds with the scene I'd carried in my head since reading the book. 'If we thought about that we wouldn't go near any book,' laughs Jarrold, whose last equally challenging project was a critically acclaimed adaptation of Dostoevsky's Crime and Punishment. 'You have to trample all over a book in order to make a good film, and simultaneously you have to remain true to the spirit of the book. I think we've succeeded in doing both here.'

On the television screen, White Teeth unfolds as a thing of often surreal and impressionistic beauty, its style and pace redolent of the successful TV treatment of Hanif Kureishi's The Buddha of Suburbia at the start of the last decade. It features a strong cast that includes the mesmeric Om Puri as Samad, and Phil Davis as his wartime pal, Archie, as well as strong performances from relative newcomers like Naomie Harris (as Clara) and Christopher Simpson (who plays both the twins, Millat and Magid). Unlike the book that spawned it, though, White Teeth the TV drama is neither reflective nor brimming to overflowing with a colourful cast of characters. Many, indeed, have disappeared altogether, or been consigned to fleeting roles either through dramatic necessity, or the demands of the £3.5m budget.

Smith, for her part, is blissfully unconcerned about the end result. 'I told them I didn't really mind what they did. As soon as I put a full stop on a book, it's not my thing any more. It ceases to be mine even more when the reader picks it up. With television, you just have to trust in the skill of the director whose job is my idea of a nightmare.'

I had heard that Channel 4 had asked her to be an advisor on the project. 'Yes, but I said no right off. Novelists don't know anything about TV. It's all about the TV moment and making those moments count totally. On TV, you can't ponce around for half an hour in the middle of a drama trying to explain the history of Jamaica.' When Smith saw some rushes, she admits to feeling 'a thrill of recognition and a kind of low-level trepidation. I saw the party scene where Clara comes down the stairs and Archie meets her, and I did feel it had that real, contained power that film has to suggest so much with just the briefest glance or look. That is something a novelist just can't do.'

Did the characters on screen chime with the characters she had created in her head? 'It may seem strange but I don't know if I had a strong visual sense of the characters at all. I mean, Nabokov is my idea of a very visual writer, but I tend to write from ideas alone.'

Nevertheless, White Teeth is a novel that, in places, has the rhythm and style of TV drama: certain scenes open and close almost as if she had the screen in mind when writing them. In the aforementioned scene in the book, where Clara descends the stairs to meet Archie for the first time, she does so complete with directorial directions - 'in slow motion, surrounded by afterglow and fuzzy lighting'. Elsewhere, as Smith admits, the novel 'could be condemned in certain places for having a sitcom aesthetic.

'It's weird, but often I feel when I'm writing certain characters and their traits that I have already seen them on TV. In sitcoms or whatever. The humour, I suppose, is TV humour to a great extent, absorbed from the usual suspects - Python, Pete and Dud and the rest. Plus, everything is always moving towards a certain point, and that's a very TV thing. A lot of American writers, like Jonathan Franzen or David Foster Wallace, are aware of these traits and go out of their way to avoid them.

'I mean, if you want to be brutal about it, a novel like, say, Updike's Rabbit at Rest is life, but White Teeth is TV. It's just my generation, I guess.'

In other ways, too, her big exuberant story, with its extended cast and overlapping stories, is tailor-made for adaptation. The terrain she maps out is both a recognisable, and at times, utterly idealised version of a present-day multiracial London populated by drifters, dreamers, losers and fanatics. Perfect, in fact, for the small screen. More problematic is its narrative which hinges on chance meetings, the crisscrossing paths of its myriad characters, and the small everyday choices and evasions that they make which can often enrich or derail a life. It is a big book about small things.

Central to this notion, and to the arc of the book itself, is the undemonstrative figure of Archie Jones, a man who has raised inaction to an art. His is the first and last voice heard in the novel, which opens with him attempting to take his own life by running exhaust fumes into his sealed car and closes with him inwardly cheering on a fleeing mouse with the words, 'Go on, my son!' - one of those English working-class exclamations that is both pared down and utterly expressive. Sadly, they are among the most expressive words Archie speaks in the book. He is thwarted even in his attempt to end it all by an irate halal butcher incensed that he has stolen his parking space.

'Archie is essentially a novelistic character in that he exists almost exclusively in his head,' Jarrold elaborates. 'He is utterly passive, which is against all the rules of film. A screenwriter would not dare to create a character like that. Put it this way, if White Teeth had been made into a Hollywood movie, I doubt Archie would have survived - the execs would have got rid of him. Yet he's integral to the story.'

'Archie represents the old England: stale, scared, unchanging, like one of those grey, drab Sunday afternoons in the 50s,' elaborates Phil Davis, the actor who plays him and whose CV stretches back to Quadrophenia and beyond. 'What's really extraordinary is that he just doesn't change. He's the same Archie at the end of the book as at the start. Samad (his best friend from the war) is his saviour; he's this wonderful enthusiast for life who hits England like a brick wall at 60mph and comes adrift. Clara (his much younger wife) is similar. These characters just bump into each other and stick.'

Given that it is such a passive role, what made Davis take it in the first place? 'I wanted to play against type for once. Plus I wanted to get under the skin of a character that I recognised. There are parts of my father in Archie. He's a product of another era, when things were more fixed and certain, but most people were essentially unhappy, trapped in awful jobs. In many ways, that's what's so lifelike about the book. It says, "This is what life is like for most people" - random, mundane, only occasionally inspiring. If you ask me, that's what touches people about White Teeth.' He stops to think for a while, looking pained and all at sea in a way you imagine Archie looking throughout the book.

Then he says something utterly un-Archie-like and totally on the money. 'As a drama, White Teeth is very much of its time. It doesn't have an axe to grind like the Alan Bleasdale or Alan Clark stuff that I grew up on. It's not about changing the world, it's about reflecting the world. It says: "We're all refugees of one kind or another, even those of us who were born here. We're all in this together." It's worth remembering that right now.'

White Teeth: the loafer's guide

According to the Whitbread Prize judges, White Teeth was 'perhaps the best novel we have read about contemporary London'. Zadie Smith, though, was altogether less generous in appraising her best-selling debut. 'The book is OK,' she once remarked, 'but it's the product of my adolescence.' The reading public seemed to subscribe to the former view.

Set mainly in Willesden, north London, where Smith was brought up, with brief forays to Bangladesh and Bulgaria, it follows the interlinked fortunes of two families, one Anglo-Jamaican, one Bangladeshi Muslim. The narrative spans 1975 until the present day, though there are leaps back to the Second World War, where Samad and Archie served together, and even further back to the Indian Mutiny.

Critics have tended to focus on the book's multicultural aspect, but, as Smith herself has said, 'I wasn't trying to write about race, I was trying to write about the country I lived in.' For this reason, perhaps, she struck an extraordinary chord in her readers, who seemed to recognise and empathise with the England she created.

Though the book's plot takes in religious fanaticism - both Christian and Muslim - as well as millennial conspiracy theorists and genetic engineering, it is essentially an old-fashioned narrative about the enduring and difficult bonds of family and friendship - bonds that are both complicated and strengthened by the often uneasy coming together of cultures and races.

Archie's courtship of the wayward Clara, on the run from a Bible-thumping mother, or Samad's doomed affair with his children's teacher, Poppy Burt-Jones, are messy, disjointed relationships played out against a backdrop of a fast-changing London, both familiar and oddly confusing. Like life, in fact.

Samad (Om Puri)

Alsana's husband, father of twins Magid and Millat, and Archie's best friend from the war, Samad is a troubled Muslim who works as a waiter and frets about the meaning of (his) life in O'Connell's pub.

Clara (Naomie Harris)

Daughter of Hortense, girlfriend of Ryan Topps, then wife of Archie, Clara is a woman who drifts into unlikely liaisons. Jehovah's Witness, Mod, then housewife, she is, says Harris, 'a person that things just happen to'.

Young Magid

(Jacob Scipio) Millat's twin brother is a boffin dispatched back to Bangladesh after witnessing his father, Samad, kiss his school teacher, Poppy Burt-Jones.

Archie (Phil Davis)

Passive to the point of suicidal at the book's start, Archie meets Clara and starts a new life much like his old life. A deadpan foil to Samad, whose ruminations, like life itself, seem to pass Archie by.

Alsana (Archie Panjabi)

Samad's long-suffering wife works at home 'sewing together pieces of black plastic for a shop in Soho called Domination', but loves England and all its institutions - 'Princess Anne, Blu-Tack... Michael Fish'.